I would check for spyware. There seem to be many that set your clock, "Precision Time" comes to mind.
Also, if it is XP it may be doing it on its own. Use net time to set it to tick.usno.navy.mil or tock.usno.navy.mil
Code:
net time /setsntp:tick.usno.navy.mil
Code:
The syntax of this command is:
NET TIME
[\\computername | /DOMAIN[:domainname] | /RTSDOMAIN[:domainname]] [/SET]
[\\computername] /QUERYSNTP
[\\computername] /SETSNTP[:ntp server list]